1. Field of the Invention
The invention pertains to the field of ship inertial navigation, and more particularly to the application of external measurements to correct errors in the predicted position provided by an inertial navigation system.
2. Description of the Prior Art
Periodic external position information is required to correct errors in the predicted position given by a ship's inertial navigation system. These errors are caused by inaccuracies that are inherent in the inertial navigation systems comprising the overall navigation system. One source of position information is the Classical Bathymetric Fix (Classical Fix) which requires the ship to transit a pre-surveyed area, known as a Precise Bathymetric Navigation Zone (PBNZ), maintain constant speed and heading while traversing the area, and produce pulsed acoustic signals at a rapid repetition rate. Each PBNZ must meet bottom slope and along and cross track variation requirements. Because of these requirements, PBNZs are only available sparsely throughout the oceans of the world. To implement the Classical Fix the nearest PBNZ to the ship must first be identified and the ship navigated to it. When the ship arrives at the PBNZ the rapid repetition rate acoustic radiation (sonar transmissions) commences. The sonar must first obtain and retain bottom lock-on and second collect sufficient depth data for position fix computation. Sonar transmissions continue for both bottom tracking and depth data collection as the ship traverses the length of the PBNZ. As the ship traverses the PBNZ it is required to maintain constant speed and heading. After the ship has traversed the entire PBNZ, the sonar depth data collected is used with previously collected and processed "truth" map data, representative of the depths in the PBNZ, in a map-matching algorithm. The differences between the two sets of data are resolved into latitude and longitude corrections which are applied to the navigation system.